The invention concerns a magnetic resonance imaging method with which an image of an object, located in a homogeneous magnetic field, is obtained. An initial projection is taken by applying a gradient magnetic field having a predetermined direction .phi. and strength G, irradiating a high frequency excitation pulse and, following a waiting time t.sub.w and with a predetermined sampling rate, sequentially measuring points of a nuclear resonance signal from the object. The signal dephases under the influence of the gradient magnetic field G.phi. and the measuring points are assigned to points in k-space lying along a vector extending from the origin. The direction of the vector is determined by the direction .phi. of the gradient magnetic field, and the separation of each of the measuring points from the origin is given by the product of the strength of the gradient magnetic field and the time interval between the excitation pulse and the taking of the corresponding measuring point. Additional n-1 (n&gt;&gt;1) projections are then taken after changing the direction and/or strength of the gradient magnetic field and the excitation, waiting time t.sub.w, and segmental measurements are repeated (n-1) times. An image of the object is constructed from the measuring points of all n projections using a reconstruction algorithm.
A method of this type is, for example, known to those of skill in the art as the so-called projection reconstruction method (back projection) and now constitutes basic knowledge in the area of magnetic resonance (see for example the textbook "Nuclear Magnetic Resonance Imaging in Medicine and Biology" by P. G. Morris, Oxford Science Publications, Clarendon Press, Oxford, 1986, .sctn.4.1).
A method is known in the art from the article "SPI-Single Point FID Imaging" by A. Nauerth and B. Gewiese, conference contribution to the .sup.12.sup.th Annual Scientific meeting of SMR, 14th-20th August 1993, New York, p. 1215, with which precisely one measuring point is taken after each high frequency excitation so that each point in k-space corresponds to one excitation ("Single Point Imaging" =SPI). The applicant's subsequently published German patent applications P 42 9 610.8, P 42 32 731.8 as well as P 43 34 038.5 likewise concern the so-called "SPI method" or variations thereof.
In the conventional imaging method the measurement signals are generally taken by measuring and digitizing a spin echo or a gradient echo signal following the high frequency excitation. Since one first allows the NMR signal to dephase following excitation and to rephase with the assistance of a 180.degree. pulse or through gradient inversion, one avoids problems associated with the fact that, directly following excitation, the receiver is overloaded and a certain minimum amount of time t.sub.w must be waited before switching from transmission to reception. For this reason, when using the original signal ("Free Induction Decay"=FID), the initial NMR signal portion is not accessible to measurement. However, neglecting this signal portion leads to enormous base line problems when Fourier transforming which renders good image reconstruction impossible. The echo signal solution is an elegant one and has significant advantages. However, this method increases the time interval between excitation and the taking of data which, in particular with investigational objects having short relaxation times T.sub.2, limits its applicability. Towards this end, the SPI method offers an alternative with which one can work with the shortest of intervals. However, this advantage is at the extreme expense of the total measuring time, since each single point in k-space must be measured individually. This cannot, in particular, be tolerated with three dimensional objects and/or biological or living samples.
It is therefore the object of the present invention to present a method which, with acceptable total measuring time, exhibits a shortened time interval between excitation and measurement compared to the echo measurement while nevertheless taking advantage of measuring points with reduced dephasing after the excitation pulse, i.e. at small k-values, for image reconstruction.